The present invention relates to a method and system for desulfurizing gasoline, diesel fuel or like hydrocarbon fuels so as to reduce the sulfur content of the fuel and render the fuel more desirable for use in a mobile vehicular internal combustion engine. More particularly, the desulfurizing method and system of this invention are operable to reduce the amount of organic sulfur compounds found in gasoline to levels which will not cause undue corrosion to engine and exhaust components. Another advantage to use of the sulfur-depleted gasoline fuel is the increased efficiency and useful life of the catalytic converters used to scrub IC engine exhaust. The method of this invention involves the use of a nickel reactant bed which has an extended useful life cycle due to the presence of oxygenates such as alcohols, water, or other compounds in the fuel stream.
Gasoline, diesel fuel, and like hydrocarbon fuels are useful as a fuel for internal combustion engines, despite the existence of relatively high levels of naturally-occurring complex organic sulfur compounds in the gasoline or diesel fuel. The sulfur compounds are undesirable since they are known to cause corrosion damage components of the internal combustion engine system, such as engine cylinder walls and exhaust system walls when the fuel is combusted. As noted above, catalytic converter performance is also adversely effected. The sulfur compounds in the aforesaid fuels are also undesirable since they are converted to sulfur dioxide (SO2) when the fuel is combusted. It is well known that SO2 and SO3,when exhausted into the atmosphere will cause xe2x80x9cacid rainxe2x80x9d due to its subsequent conversion to H2SO3 and H2SO4 in the ambient atmosphere. The former problem of engine damage has not been addressed in any fashion, other than by attempting to reduce the amount of sulfur in the gasoline or diesel fuel during the refining process. This solution also helps ameliorate the exhaust problem, but the acceptable amount of sulfur compounds in gasoline or diesel fuel is not consistent from state-to-state. At the present time, California has the most stringent requirements for fuel sulfur content, which is about 30 ppm sulfur in the fuel. Even with this low concentration of sulfur in the fuel, engine damage and decreased catalytic converter performance can still result.
An article published in connection with the 21st Annual Power Sources Conference proceedings of May 16-18, 1967, pages 21-26, entitled xe2x80x9cSulfur Removal for Hydrocarbon-Air Systemsxe2x80x9d, and authored by H. J. Setzer et al, relates to the use of fuel cell power plants for a wide variety of military applications.
It would be highly desirable from an environmental standpoint to be able to power vehicles, such as an automobile with a low sulfur fuel, such as a low sulfur gasoline or diesel fuel. In order to provide such a vehicular power source, the amount of sulfur in the processed fuel gas would have to be reduced to and maintained at less than about 0.05 parts per million. The desulfurized fuel can be used as a fuel for an internal combustion engine. The fuel being processed can be gasoline or diesel fuel, or some other fuel which contains relatively high levels of organic sulfur compounds such as thiophenes, mercaptans, sulfides, disulfides, and the like. The fuel stream is passed through a nickel reactant desulfurizer bed wherein essentially all of the sulfur in the organic sulfur compounds reacts with the nickel reactant and is converted to nickel sulfide leaving a desulfurized hydrocarbon fuel. Previously filed U.S. patent applications Ser. No. 09/104,254, filed Jun. 24, 1998; and Ser. No. 09/221,429 filed Dec. 28, 1998 generally describe systems for use in desulfurizing a gasoline or diesel fuel stream for use in a mobile fuel cell vehicular power plant; and in an internal combustion engine, respectively.
We have discovered that desulfurization of a gasoline or diesel fuel which uses a nickel catalytic adsorbent bed cannot be performed over a significantly extended period of time, unless the fuel includes an oxygenate compound in appropriate proportions, a small amount of added water, preferably in the form of steam. or a small amount of added hydrogen. Various oxygenates which could suffice for the desulfurization process include MTBE, ethanol or other alcohols, ethers, or the like.
This invention relates to an improved method for processing a gasoline, diesel, or other hydrocarbon fuel stream over an extended period of time, which method is operable to remove substantially all of the sulfur present in the fuel stream.
Gasoline, for example, is a hydrocarbon mixture of paraffins, naphthenes, olefins and aromatics, whose olefinic content is between 1% and 15%, and aromatics between 20% and 40%, with total sulfur in the range of about 20 ppm to about 1,000 ppm. The national average for the United States is 350 ppm sulfur. The legally mandated average for the State of California is 30 ppm sulfur. As used in this application, the phrase xe2x80x9cCalifornia Certified Gasolinexe2x80x9d refers to a gasoline which has between 30 and 40 ppm sulfur content, and which contains about 11% by volume MTBE at the present time. California Certified Gasoline is used by new car manufacturers to establish compliance with California emissions certification requirements.
We have discovered that the presence of oxygenates in the gasoline, like MTBE (methyl-tertiary-butyl ether, i.e., (CH3)3COCH3), ethanol, or water vapor for example, will prevent rapid deactivation of the nickel catalytic adsorption of organic sulfur compounds from the fuel stream. Ethanol could be an appropriate solution to this problem since it is non-toxic, is not a carcinogen, and is relatively inexpensive and readily available in large supplies as a byproduct of the agriculture industry. Methanol, which would also extend the desulfurizer bed life, is not preferred since it is toxic; while MTBE is likewise not preferred since it is thought to be a carcinogenic compound, and may be banned in certain areas of the United States in the near future by new environmental regulations. Preferred oxygenates are non-toxic and non-carcinogenic oxygen donor compounds, such as ethanol, water vapor, or the like. When water is an oxygenate included in the gasoline or diesel fuel mixture being desulfurized, the water content of the fuel mixture should be in the range of about 2% to about 5% by weight of the fuel mixture.
The effectiveness of a nickel adsorbent reactant to adsorb organic sulfur compounds from gasoline depends on the relative coverage of the active reactant sites by adsorption of all the various constituents of gasoline. In other words, the catalytic desulfurization process depends on the amount of competitive adsorption of the various constituents of gasoline. From the adsorption theory, it is known that the relative amount of adsorbate on an adsorbent surface depends primarily on the adsorption strength produced by attractive forces between the adsorbate and adsorbent molecules; secondarily on the concentration of the adsorbate in the gasoline, and temperature. Coverage of a reactant surface by an adsorbate increases with increasing attractive forces; higher fuel concentration; and lower temperatures. Relative to gasoline, Somorjai (Introduction to Surface Chemistry and Catalysis, pp, 60-74) provides some relevant information on the adsorption of hydrocarbons on transition metal surfaces, such as nickel. Saturated hydrocarbons only physically adsorb onto the nickel reactant surface at temperatures which are less than 100xc2x0 F., therefore paraffins, and most likely napthenes, won""t compete with sulfur compounds for adsorption sites on the nickel reactant at temperatures above 250xc2x0 F. and 3000xc2x0 F.
On the other hand, unsaturated hydrocarbons, such as aromatics and olefins, adsorb largely irreversibly on transition metal surfaces even at room temperature. When an unsaturated hydrocarbon, such as an aromatic or an olefin, adsorbs on a transition metal surface, and the surface is heated, the adsorbed molecules, rather than desorbing intact, decompose to evolve hydrogen, leaving the surface covered by partially dehydrogenated fragments, i.,e., tar or coke precursors. We have discovered that, at 3500xc2x0 F., unsaturated hydrocarbons are nearly completely dehydrogenated, and the dehydrogenated tar fragments form multiple carbon atom-to-nickel reactant surface bonds. This explains why aromatics and olefins in gasoline, in the absence of oxygenated compounds in appropriate concentrations, will deactivate the nickel reactant from adsorbing sulfur after a relatively short period of time.
In general, the adsorption strength of a compound depends on the dipole moment, or polarity, of the molecule. A higher dipole moment indicates that the compound is more polar and is more likely to adsorb on a reactant surface. Aromatics are an exception to this rule because their molecular structure includes a xcfx80 ring of electron forces that produces a cloud of induced attractive forces with adjacent surfaces. Based on the dipole moments of hydrocarbons, allowing for the xcfx80 ring in aromatics, the order of adsorption strength (highest to lowest) is: nitrogenated hydrocarbons greater than oxygenated hydrocarbons greater than aromatics greater than olefins greater than hydrocarbons containing sulfur greater than saturated hydrocarbons. Since the adsorption strength of the oxygenated hydrocarbons (such as ethanol, methanol, MTBE, or the like) is greater than that for aromatics and olefins, oxygenated hydrocarbons, or other oxygen donor compounds, if present in the gasoline or diesel fuel being desulfurized, will provide greater coverage of the nickel reactant sites than do the aromatics and olefins in the gasoline. Thus, the oxygenated hydrocarbons can reduce the adsorption of aromatics and olefins on the nickel reactant bed. Although saturated hydrocarbons (paraffins and cycloparaffins) would not be expected to be adsorbed on the desulfurization nickel reactant to a significant extent, oxygenated hydrocarbons will also prevent them from adsorbing onto the nickel reactant.
We have also discovered that the adsorbed oxygenated hydrocarbons do not inhibit the sulfur compounds from being adsorbed on the nickel reactant. The oxygenated hydrocarbons and the sulfur compounds are both quite polar and therefore they are miscible, which allows the sulfur compounds to dissolve into and diffuse through the adsorbed layer of oxygenated hydrocarbons to the active nickel metal reactant sites. Thus, the oxygenated hydrocarbons provide a xe2x80x9cshieldxe2x80x9d which inhibits the carbon-forming hydrocarbons from contacting the nickel reactant sites while allowing the sulfur compounds to contact and react with the active nickel metal reactant sites.